1. Field of the Invention
The present invention generally relates to apparatuses for and methods of generating a timing signal more particularly, the invention is to an apparatus for and a method of generating a timing signal to control rotating speed of a digitally controlled actuator such as a stepping motor. More specifically, the present invention relates to an apparatus for and a method of generating a pulse signal to control rotating speed of a motor which drives running of a recording medium, such as a magnetic tape used in an audio tape or video tape recording and reproducing apparatus.
2. Description of the Background Art
In a video tape or audio tape recording and reproducing apparatus, a motor is required to drive a magnetic tape serving as a recording medium. In applying a capstan motor or the like to this purpose, a DC motor has generally been used in general. However, a DC motor can be easily affected from fluctuation in drive current, so that in order to continue precise rotation in a motor at a desired speed, a complicated servo control circuit is required.
Meanwhile, it has become common in the electronics art to perform signal processing digitally with a view to facilitating and precise signal processing, a simplified circuit structure and the like. In line with this tendency, a digital servo system which controls digitally rotating speed and phase of a motor has been employed also in the recording and reproducing apparatuses as described above Furthermore, for a tape running driver motor, a stepping motor which can be controlled digitally in rotating speed and phase has come into use. The use of such a digitally controllable stepping motor enables a magnetic tape as recording medium to be well-controlled for running steadily at a desired constant speed, resulting in high-quality recording or reproducing.
In driving such a stepping motor (pulse motor) as a digitally controllable actuator, it is necessary to generate and apply to the motor a plurality of pulse signals depending on its driving method, for example, signals of the same frequency, and of different phases. The stepping motor is driven to rotate synchronized with these pulse signals.
In an audio tape or video tape recording and reproducing apparatus which employs such a stepping motor as a magnetic tape running driver motor, there is provided a timing signal generating circuit for generating a motor driving pulse signal or a timing signal, as shown in FIG. 1.
Referring to FIG. 1, a conventional timing signal generating circuit 1 comprises a read only memory (ROM) 100 for storing fixedly timing signal patterns in the form of a table, a clock generating circuit 8 for generating a pulse signal of a predetermined period, or a clock signal, a counter 9 for counting the clock signal from the clock generating circuit 8, and a control circuit 101 for generating a timing signal (drive pulse) of a predetermined pattern based on count value of the counter 9 and stored information of the ROM 100.
The ROM 100 stores time information (cock signal number) and patterns of the timing signal to be generated corresponding to the time information.
A timing signal from the control circuit 101 is applied to a drive circuit 15. The drive circuit 15 is responsive to an applied timing signal for driving a stepping motor 16 which may be, for example, a capstan motor. The motor 16 drives a recording medium which may be, for example, a magnetic tape (referred to simply as magnetic tape hereinafter) 50 to run through the rotation therefrom. In the following, operation will be described in brief.
It is here assumed that the motor 16 is a one-phase-on drive system three-phase stepping motor. The ROM 100 stores drive pulse generating patterns as shown in FIG. 2 together with time information in the form of a table. The time information is stored in the form of a number of clock signals. For example, during the time from t0 to t1 a drive pulse signal .phi.A is generated, so that information of (t, .phi..sub.A, .phi..sub.B, .phi..sub.c)=(t0, 1, 0, 0) is stored.
Now, assume that a motor rotation start instructing signal is applied to the timing signal generating circuit 1. The control circuit 101 is responsive to this motor rotation start instructing signal for reading out time information (clock signal number n0) at a first address in the ROM 100 to compare the read-out time information with count value of the counter 9. When match is found between this read-out time information and the count value of the counter 9, the control circuit 101 reads out the pulse pattern information corresponding thereto from the ROM 100 and applies the same to the drive circuit 15. At this time, only the drive pulse signal .phi..sub.A is represented "1" ("H"), in response to which the motor 16 begins to rotate therein, causing the magnetic tape 50 to run. Thereafter, the control circuit 101 reads out the subsequent time information t1 from the ROM 100 to compare it with the count value of the counter 9. The timing signal (drive pulse) read out at the time t0 continues to be applied to the drive circuit 15 until another match is detected. Upon reaching the time t1 (clock signal number n1), the corresponding timing signal is read out of the ROM 100 and applied to the drive circuit 15. This causes running of the motor 16 in response to the drive pulse .phi..sub.B. By repeating this operation, the drive pulse signals .phi..sub.C, .phi..sub.A and .phi..sub.B are sequentially applied to the motor 16 at predetermined intervals.
In this structure that the clock generating circuit 8 is constituted of a crystal oscillator or the like, precise time information can be derived so that stable rotation in the motor 16 precisely at a desired speed is obtainable. Accordingly, it becomes also possible to stably drive the magnetic tape 50 with high precision.
Meanwhile, as the present-day audio tape or video tape recording and reproducing apparatuses are provided with higher functions, diverse rotating speeds are required for the motor. The video tape recording and reproducing apparatuses are, for example, provided with various modes such as fast reproduction, variable slow reproduction and reverse reproduction as well as normal reproduction, so that it is necessary to change the rotating speed in the magnetic tape running driver motor, corresponding to the respective modes.
However, in the conventional timing signal generating circuit, the data for producing timing signals which define rotating speed in the magnetic tape running driver motor are stored fixedly in a small-capacity ROM. Therefore, the variety of output patterns thereof is limited.
Accordingly, in implementing a multi-functional video tape recording and reproducing apparatus with the use of the conventional timing signal generating circuit, a plurality of timing signal generating circuits are required, and this may lead to a more complicated apparatus structure and thus an increased cost therefor.
In this case, while storing all possible output patterns in the ROM allows the apparatus to accommodate to various rotating speeds for the motor, unnecessary or not used output data patterns are also stored in the ROM for some types of the applied apparatus so that the ROM is unnecessarily increased in capacity and the utility efficiency of memory area in the ROM is reduced as well.
Furthermore, even if all of the output data patterns stored in the ROM are used, there are also stored those output data patterns whose utility frequency is low. This means reduced utility efficiency of the memory area in the ROM, to be a factor of an increased cost.